Certificate error using CoursePress Pro

Hello,

I have added an image as a background to certificates in CoursePress Pro. I have tried a number of different images and get the same result. When a student click certificate there is an error message

TCPDF ERROR: [Image] Unable to get image: http://learnsocial.media/wp-content/uploads/2016/02/dreamstime_xl_12010865.jpg

Any ideas?

Thanks

Andrew